Manufacture of sulphuric acid



Patented Apr. 30, 1940 ujPATE MAnUrAo'rURE F SULPHURIQ Aom William E.Watson, Nutley, N. J assignor toGeneral. Chemical Company, New York, N.Y.,. a H corporationof New York l No firawing. Application March 20,1939, i o Serial No. 263,051

8 Claims.

This invention relates to purification of sulphuric acid. Sulphuric acidsolutions such as spent nitrating acid, the residual sulphuric acid ofnitric acid concentrating and drying operation, and Glover tower acidcontain variable relatively small amounts of contaminatingnitrogenoxygen compound impurities generally understoodin the art to bein the form'of nitric acid as such or as nitrosyl sulphuric acid. Theinventionis more particularly directed tomethods for treatingordenitrating sulphuric acid soluohs of "thetyp'e mentioned toeffectremoval of such impurities. I Prior denitrating methods include. blowingthe w sulphuricacid solution to be treated with air,

steam orlother gases, nitrogen impurity removal in these instances beinglargely mechanical. The more common and most satisfactory priorcommercial method involves addition of ammonium,

However, use of ammonium sulphate for this pur- The nitrogen-oxygencompound impurities. in

acid; When using ammonium sulphate according to-prior practice under themostfavorable conditions, i; e., treating an impure sulphuric acidsolution of H2804 concentration of around 75-71% and at averagetemperature of about 300 l t, the reaction is very slow and ordinarilyrequires from eight to twelve hours at such temperature to go tocommercially satisfactory completion. When the nitrogen-oxygen compoundimpurity is in the form of nitric acid as such, e; g., in the case ofresidual sulphuric acid ofa nitric acid concentrating and dryingoperation,

thereactionis still slower and at temperature of about 300 Fl, even withlonger treatment Or re- 40 tention time andwith use of excessivequantities of ammonium sulphate satisfactory results are not obtained.While in some plant operations the time element itself may be of nomaljorimportance, it will beunderstood thatwhere long timeintervals' arerequiredto effect desired completion of. the denitrating operation it.ishecessary tosupply substantialamount of extraneous heattokeep thetemper'atureof the sulphuric 5 acid solution undergoing treatment up tothe most favorable temperatures for the reaction.

Another important disadvantageof use of ammonium sulphate is that whenconcentration of; thesulphuric acid solution being treated isappreciablyin excessof about -77%; regardless sulphate to the sulphuricacid to be denitrated.

Glovertower acids and in spent nitrating acids are considered to bepresent as nitrosyl sulphuric of Whether the nitrogen impurity ispresentas nitrosyl sulphuric acid or nitric acid as such, thepurification reaction slows down to such an extent as to. be of littleor no commercial value.

The primary object of this'invention is to pro- 5 vide an improvedmethod for denitrating sul phuric acid solutions contaminated withnitrogen-oxygen compound impurities. I

I have discovered that under certain conditions of temperature andoil-128 04 concentration of the sulphuric acid solution beingdenitrated, urea efiects very rapid removal of nitrogen-oxygen compoundimpurities of the type which contaminate sulphuric acid. I have foundthat not only does use of urea make possible removal of nitrogen-oxygenimpuritiesto any desired degree of denitration, but also has theadvantage of substantially wideningthe permissible H2303 concentrationof the sulphuric acid solutions being treated, and greatly reduces thetime interval during which the acid mass being treated must be retainedat treatment temperatures. Further, notwithstanding the pound for poundincreased cost of urea as compared with ammonium sulphate, I find itpossible to use such appreciably smaller amounts of urea that the costof the purification operation as a whole is substantially reduced. y Theinvention is applicable to denitration of sulphuric acid solutionscontaining as impurities nitrogen-oxygen compounds in any form which maycontaminate such solutions. indicated, the three most common types ofcommercial sulphuric acid solutions which may be denitr'ated toparticular advantage according to the invention are the residualsulphuric acids of nitric acid concentrating and drying operations,spent nitrating acids, and Glover tower acids. It is not with certaintyknown in what'form the nitrogen-oxygen impurities exist inxsulphuricacid. solutions of these characteristics, but it is the consensus ofopinion I in the art that in the cases of spent nitrating acids andGlover tower acids, the nitrogenis present as nitrosyl sulphuricacid,probabilities being that any lower oxides of nitrogen areconvertedby. sulphuric acid to nitrosyl sulphuric acid. As tothe residualsulphuric acid solutions of nitric acid drying and concentratingoperations, it appears that the nitrogen-oxygenim x purity is present asnitric acid. The process of I the invention includes purification ofsulphuric. acid solutions containing any amounts of nitrogen-oxygencompound impurities in whatever NT OFFICE about 95%. Accordingly, it ispreferred to operate with these concentrations. "Although the principlesof theinventionare applicable and substantially improved results maybeobtained when using HzSOu concentrations as lowas and as high as 99.5%,the nitrogen impurity elimination reactions take place more slowly atconcentrations between 60% and and between and 99.5%. Indications are.that good results depend at least to some extent upon the presence inthe'solution being treated of some "amountof water. The sulphuric acidsolutions to be denitrated if not alreadyof the I-IzSOe concentrationsindicated may be brought within the desired operatingrang-e in anysuitable way'a's by concentration orby dilution with water.

It is noted'thebulk of commercial sulphuric acid solutions which maybedenitrated' accordingto-the invention, such as spent nitrating acids,

Glover tower acids and residual sulphuricacid solutions of nitric acidconcentrating and drying operations, are discharged from the respectiveprocesses inwhich such sulphuric acid solutions are formed as sought forproducts or by-products, at H2804 concentrations above 60% and in manyinstances within an H2804 concentration range of '70 to about 85%.Accordingly, it will be seen that in present commercial operations towhich the process of the invention is most applicable, the sulphuricacid solutions to be denitrated are already of suitable H2804concentration and no preliminary treatment of the sulphuricacid solu-.

tionsto be denitrated is necessary for'the' purpose of bringingsuchsolutions within desirable H2804 concentration ranges of theinvention.

As to temperature, I have discovered that during the" course 'of'thepurificationreactions the temperature of the acid mass undergoingtreatment should .be kept at not less than F. Practice of the inventionshows that within reasonable limits the higher the temperature, the

more effective is removal of contaminating nitrogeneoxy gen impuritiesand the shorter the reaction time. The upper temperature limit is onemore cr less of economics, and practice of embodiments .of the inventionindicates that while temperatures above about 350 F. may be employed,temperatures above 350 F. are unnecessary and afford noparticularpractical advantages. As to best results with respect tosubstantially complete elimination of contaminating nitrogen-oxygenimpurities and minimum time intervals within which thedenitrationoperation is comp1ete,.I find temperaturesof not less thanabout 300 F. are most desirable. been discovered that satisfactorycommercial results may be secured when operations are such thattemperatures during thedenitration: operation are of the order of notless than 240 F., 2.1- though at temperatures" between say 240 F; and

-,300 F. while overall nitrogen removalis equally efficient the timerequired for the nitrogen elimi-' nation reaction to take placeisslightly greater:

It has also and becomes more prolonged as temperatures decrease. I

In commercial procedures resulting in formation of sulphuric acidsolutions which require denitration such as Glover tower acids andsulphuric acid solutions used in nitric 'acid concentrating and dryingmethods, operations are such that these sulphuric acid solutions aredischarged from their respective processes at-temperatures above about240 F.. Applying the present invention to denitration of thesecommercially common sulphuric acid solutions (which as indicated abovehave I-IzSOiconcentrations within the previously described limits) Ifind that when the .initial temperature of these or other sulphuricacid. solutions at the time of introduction of urea is about 240 F. orabove, the nitrogen elimination reactions take place so fast that noconsideration need be given to the normal temperature drop'ofthe acidbody since long before the acid body will have normally cooledtolessthan 140 F, the nitrogen removal reactions will: have taken place.This is a marked operating advantage afforded by the invention, since byreason of the rapidity of completion of nitrogen elimination it isunnecessary, when working commercially operations of the type mentioned,to use any ex; traneous heat to keep up the sulphuric acid 'solii tionbeing denitrated up to optimum tempera; tures for treatment. Othersulphuric acid solu-* tions which may be normally at temperatures lessthan the treating temperatures indicated may be brought to and kept atproper temperatures by any suitable means. I

The amount of urea to be used in any Lpartidu:

lar operation is dependent upon fourdifferent factors all of which aresubject tofairly Wide variation: namely, (1) the H2804 concentration.

' of denitration.

These variable conditions are discussed in general as follows. Thus,other factorsbeing the same, the more remote the H2804 concentrationfrom the preferred H2804 concentration range, the greater is the amountof urea needed. Other factors beingthe same, the lower the tempera-'-ture, the greater the amount of urea "required: Ihe larger the amount ofcontaminating nitro gen impurities contained in a givensulphuric acidsolution, other" factors being the same, the more urea should be used.Further, the shorter the permissible time interval within which it isdesired to complete reaction, the more urea is needed. 0n the otherhand, where' the H2804 concentration and temperature are favorableftheamount of nitrogen-oxygen impurities is low, and it is not necessarytocomplete the reaction with maximum rapidity, the amountof urea needed islessened. In view of these variable conditions, each of whichin one wayor'anotherafiects'all' of the'others, as will be apparent'it is notpossible to specify definite directions with respect to the amountofurea tobe used. in all situations. However, the quantity of urea to beemployed under any given set of conditions maybe easily found. Forexample, assuming an operator-is dealing with a sulphuric acid solutioncontaining. a known amount of contaminating. nitrogen-im-; purities andof known H2804 concentratiomand it is desired to operate at a definitetemperature. 15,-

ithe projecteduoperating temperature and noting ,.-.the amount of urearequired to accomplish the desired, degree of ,denitration. within thespecified time interval; ,lnidevelopment of the invention satisfactoryresults have been obtained where urea has been used in quantitiesrangingfrom 200 to 1000 pounds (dry basis) per 120 tons of sulphuric acidsolution treated. In this work, H2304 concentrations varied from 60% to,substantially 99.5%, and nitrogen impurity content equivalent to HNOsvaried, in thesulphuric acid U solution treated, from about 0.10% toabout 0.50%. e In general, during time intervals ranging from about to105 minutes nitrogen-oxygen impurity content was reduced to from ..004%or less to usually nil. When working under the more favorable conditionswith respectto temperature and H2SO4 concentrations, ,impufrity contentwas reduced to nil in from 5 to 15minutes. i i

The reactions involved in the process of the inventionare not fullyunderstood nor is it understood why the results are obtained under thepreviously described H2804 concentration and temperature conditions.Experience in practice of the invention afiords basis for the thoughtthat where the nitrogen-oxygen compound to be removed is in the form ofnitric acid, elimination of nitrogen is effected in accordance witheither or both of the following equations: l

, actual quantity used underfavorable conditions. 9

Indications are that the reaction of Equation 1 may predominate sincewhen denitrating a sulphuric acid solution in which the nitrogen isprobably all in the form of nitric acid, calculation of, the amount ofurea required perunit of nitric acid destroyed checks very closely withOn introduction of the urea into the sulphuric 1 acid solution beingtreated, especially when operating within the preferred H2804concentration and temperature ranges there is rapid evolution of gasesand vapors from the surface of the acid mass. It seems probable thisrapid evolution of gases serves to mechanically remove somenitrogen-oxygen impurity as vapor, thus supplementing chemical reaction,and when it is desired to obtain the lowest possible consumption of ureaadequate provision shouldbe made for promptly removing the gases andvapors from the surface ofthe acid mass as soon as evolved.

The process of the invention may be carried out as a continuousoperation or batchwise, the former being preferred. The urea may, ifdesired, be introduced in solid :form although for operating conveniencea water solution of urea is recommended. One exampleof the invention,carried out as acontinuous operation, is as follows: The sulphuric acidsolution treated was the residual sulphuric acid discharged from anitric acid concentration and drying operation and contained about 75%H.280; and about 0.12% HNOc. The temperature of the acid leaving theconcentration and drying operation averaged about 300 F. Urea was usedin the'form of a 40% solution to facilitate feeding. The effluentsulphuric acid of the nitric acid concentrator was continuouslyrunwithout cooling into the first of tworetentio'n tanks connected inse'ries, land at the sametime thek40i%. urea; solution was continuouslyintroduced into acid strearic entering thefirstitank. The relativeamounts ofsulphuric acidrsolution and urea solution were so adjustedthat over the course of the operation between-4001 and 500 pounds ofdryiurea were used to treat 100 tons of residual sulphuricacid having.arixJ-IzSOiconcentration of about 75%. Under these conditionsiofxacidconcentration and temperature, my investigations show that, as far withdiphenylamine, complete elimination of nitrogen compoundsis effectedinaround 15 min utes. However, in a commercial-size continuous operationit is recommended, as was done in this example, that the sulphuric acidsolution be run through a retention zone (in this instance two tanksconnected in series) having capacity sufficient to provide for retentionof all portions of as canibe'detectedby known chemical tests, e. as

the acid solution for between one and two hours t in order to permit allof the sulphuric acid solution being treated sufiicient timeto becomewell mixed with urea. .In the example mentioned, application ofextraneous heat was unnecessary, the temperature of the solution droppedabout 10-15 F., and the HNOs content of the treated sulphuric acidsolution was reduced to nil.

I claim: I

1. 'Ihemethodfor purifying sulphuric acid. containing contaminatingnitrogen-oxygen compound impurities which comprises treating said acidwith urea to effect removal of nitrogen therefrom, said acid having anH2504 concentration of not less than about and not more than 99.5%, andmaintainingthe temperature of the acid mass during the purificationoperation at not less than about 140 F.

2.Themethod for purifying sulphuric acid containing contaminatingnitrogen-oxygen compound impurities which comprises treating said acidwith urea to, effect removal of nitrogen therefrom, said acid having an,H2804 concentrathan'about 95%, and maintaining the temperature of theacid mass during the purification pound impurities which comprisestreating said acid with urea to effect removal of nitrogen therefrom,said acid having an H2SO4 concentration of not less than about 70% andnot more than about 95%, and maintaining the temperature of the acidmass during the purification operationnot substantially less than about240 F.

5. The method for purifying sulphuric acid containing contaminatingnitrogen-oxygen compound impurities which comprises treating said acidwith urea to effect 3 removal of nitrogen therefrom, said acid having anH 804 concentration of not less than about 70% and not more than about95%, and maintaining the temperature of the acid mass during thepurification operation at not less than about 300 F 6. The method forpurifying sulphuric acid 75 tion of not less than about 70% and not morecontaining contaminating nitrogen-oxygen com- :pound impurities whichcomprises treating said acid with urea to efiectremoval of nitrogentherefrom,--'s'aid acid having an ,HzSO4-concentration of notrless thanabout and 'not'more than 99.5%., and the temperature of said acid at thetime of addition of the urea beingnot less acid with urea to effectremoval of nitrogen therefrom, said acid having an H2SO4 concentrationof not ,less than about 10% and not more than. about 95%, and. thetemperature of said acid at the time of addition of the urea being i notlessthan,aboutc240 F. v

8. The method for purifying sulphuric acid containing contaminatingnitrogeneoxygen compound impurities which comprises treating said acidwith urea toeffect removal of nitrogen therefrom, said acidhaving-anHzSOrconcentration of not less than about and not more than about andthe temperature of said 1 acid at'the time of addition of the urea beingnot less than about 300 ,F.

" w WILLIAM E. WATSON.

